It is desirable to provide compressed air and other gases at low humidity levels for use in a device or a process. One example of this is the use of compressed air in a dust collection device. Dust from a manufacturing operation is collected on the outside of a bag filter. These bag filters operate in bag houses which are generally located outside in potentially cold environments. The dust which collects on the bag surface is separated from the filter bag with the use of compressed air. If the air is high in relative humidity, the dust will be wet and be retained on the bag. This will reduce the efficiency of the bag filter. The lower the dew point of the compressed air to the bag filter, the less possibility for condensation to effect the performance of the bag house.
Many devices are known to remove water from compressed air systems. These devices can be broken down into two basic types. The first is a style of filter which relies on velocity changes and flow path interruptions to remove condensed liquids from the air stream. This style of moisture removal device has been used for years with tremendous success. An example of this style unit is a non-absorptive coalescent filter as described in U.S. Pat. No. 4,822,387. This style of unit does not, however, appreciably lower the relative humidity of the compressed air.
The other type of moisture removal device passes the compressed air through a body of material which absorbs water. Generally, the water vapor may be passed over a bed of desiccating material, such as activated alumina or silica gel. This style of filter/dryer eventually becomes saturated with water vapor and loses efficiency. The desiccant can be regenerated by heating the desiccant to a high enough temperature to drive off the collected water vapor. Desiccant dryers having a replaceable canister containing desiccant are known. Examples of this are U.S. Pat. No. 2,758,719 to Line; U.S. Pat. No. 3,353,339 to Walter; and U.S. Pat. No. 4,659,467 to Spearman. The '719 patent utilizes a central distribution tube to force wet compressed air to the bottom of a removable but disposable canister containing desiccant.
The distribution tube is rigidly held in place by permanently attached radially extending spiders attached to the shell. This type of assembly limits the means of regenerating the desiccant media and reusing the outer housing. In the '339 patent, the desiccant canister is enclosed in a chamber. The canister itself is made of two concentric tubes which are cemented or otherwise suitably affixed to two end caps. This style of canister is functional but has two design problems: the end caps being cemented do not allow for access to the filter media. The second problem is that with the internal threaded rod, a significant portion of the filtering volume is lost to internal support.
In the '467 patent, the desiccant canister design is again a disposable product, not lending itself to regenerating the desiccant bed. The housing for the bed comprises a top cap which is crimped to an outer shell to form a sealing surface. It would be more practical with this style of design to replace the entire spin on, spin off canister. It is also noted that the '467 patent includes a centrally located distribution tube for flow of wet air to dry air to utilize the entire bed of filtration media. This limits the volume of filtration media which can be installed in the canister.